Laser interferometers are used in many applications requiring precise, high resolution displacement measurements. One popular type of laser interferometer is a two-frequency, or AC, laser interferometer system.
FIG. 1 shows a typical block diagram for an AC laser interferometer system, of the type suitable for use with the fiber optic based receiver of the invention. Other optical arrangements for the interferometer are possible and could also be used with the receiver of the invention to achieve similar results. In a simple linear measurement using this system, a two-frequency laser beam is split by a remote interferometer 13, so that a component with one frequency is reflected by a fixed reflector 17, and the other component with a different frequency is transmitted toward a movable reflector 19 mounted at the point whose movement is to be measured.
When reflector 19 is moved, a doppler shift is introduced into that beam's frequency. The magnitude of this doppler shift is proportional to the speed at which reflector 19 is moving. The beams are recombined and sent to a receiver 14 which measures the difference of the two frequencies and produces a MEASURE signal.
A portion of the original laser beam is sampled and detected in a similar manner by receiver 18 to produce a REFERENCE signal. The measurement electronics 16 compares the cumulative phase difference of the MEASURE and REFERENCE signals, and the result is proportional to cumulative displacement. See, U.S. Pat. No. 3,458,259, "Interferometric System", issued July 29, 1969, and U.S. Pat. No. 3,656,853, "Interferometric System", issued Apr. 18, 1972.
Conventional laser interferometers employ remote receivers to perform frequency mixing, detection, amplifying, and cable transmission functions in a single package. One receiver is required for each measurement axis.
FIG. 2 shows a schematic diagram of a conventional integrated receiver 14, known in the prior art. The combined laser beam returning from the interferometer enters the receiver through the lens 21, then passes through the mixing polarizer 23 which mixes the two beam components, producing interference fringes. The light beam is focused onto the detector photodiode 25 which converts the light into an electrical signal. The output of the detector is applied to an amplifier 27 and a line driver 29 to produce the signal for the measurement electronics.
Such conventional integrated receivers suffer several disadvantages:
Heat dissipation. Due to the use of electronic detection, amplifying, and cable transmission commercial remote receivers generate heat that can adversely affect the nearby measurement. This effect can be reduced by mounting the receiver sufficiently far away from the measurement, but this isn't always convenient or practical.
Size and Volume. Present receivers occupy approximately 5 cubic inches. In many applications it is difficult to find the room for a receiver of this size.
EMI considerations. Present receivers employ electronic signal transmission over a cable between the receiver and the measurement electronics. In present commercial AC interferometers these signals are in the range of 100 kHz to 25 MHz. Without careful design, the signal from the receiver is susceptible to noise from ambient electromagnetic interference, especially when the receiver is separated from the measurement electronics by great distances. Also, the potential for electromagnetic interference from the receiver exists.
An object of the invention is to provide a remote receiver for laser interferometer systems that overcomes the disadvantages of conventional receivers discussed above.
In the preferred embodiment of the invention, the receiver is split into two parts connected by a fiber optic link. The front-end of the receiver, located near the remote interferometer optics, houses only the optical components for focusing and combining the laser beam components, and transmits the combined beam through an optical fiber cable. The back-end of the receiver, located remotely, houses the electronic components for detecting and measuring the frequency difference to produce the signal for the measurement electronics.
Such a receiver offers the following advantages:
No heat dissipation. Since the receiver front-end (lens, polarizer, and fiber) are all passive devices, there will be no heat generated in the vicinity of the measurement.
Small size. The receiver front-end can be packaged in a small cube about 3/4 inch on a side because it houses only the optical components.
No EMI radiation or susceptibility. Using a fiber optic cable to carry the displacement information to remotely mounted electronics allows shorter electrical cabling, reducing the system's EMI susceptibility and radiation .